I have finished my masters degree in cosmology and my research was on inflation, despite it being interesting, I'm not really fond of doing simulations/computations using a computer for a very long time. Don't get me wrong, I know that every physicists should know at least some computational tool (to some degree but not necessarily an expert) but I just don't want to work on computers most of the time. I've also thought of doing a PhD in mathematical physics but I realized that I want to be rigorous in what I do but not to the level of what mathematicians do (e.g. proving the existence of Navier Stokes theorem). So I think my intention is in between those two differing fields. I have a background on the standard undergraduate and graduate physics curriculum plus GR (Carroll) and cosmology (some basic books plus papers). I just enjoy doing more sophisticated mathematics than the average physicist would enjoy (I enjoyed studying from Apostol, Lang, Edwards, etc) but I think it would also discourage me to do for example Papa Rudin's books. I'm currently reading Mathematical Physics by Szekeres and Hassani, and I think I like the flavor of it. Can anyone recommend the path I should take and what are the current fields/topics that I can look into if it will interest me. I'm planning on doing a PhD in Europe preferably in Germany, Sweden, or Ireland.